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-- The following external packages were located on your system.
-- This installation will have the extra features provided by these packages.
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* okular - Provides an Okular backend for Mobipocket documents
* strigi libraries - Provide a strigi analyzer for Mobipocket documents

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-- Congratulations! All external packages have been found.
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The link-commandline misses "-llibokularcore", so either this is an upstream error or a problem while extracting the needed source files or patching the CMake-files. In any case you should report this at bugs.gentoo.org.

// edit: In order to get processed your report as fast as possible you should attach the COMPLETE (!) build log and your emerge --info.